1. Field of the Invention
The present invention relates generally to electrostatic discharge protection of integrated circuits.
2. Description of Related Art
An integrated circuit (IC) connected to external ports is susceptible to damaging electrostatic discharge (ESD) pulses from the operating environment and peripherals. The same ever-shrinking IC process technology that enables such high-port interconnect data rates can also suffer from higher ESD susceptibility because of its smaller fabrication geometry. Additional external protection devices can violate stringent signaling requirements, leaving design engineers with the need to balance performance and reliability.
The changing application environment is also contributing to increased ESD vulnerability. A proliferation of laptop computers and handheld devices such as cell phones, personal digital assistants (PDAs), and other mobile devices are being used in uncontrolled environments (i.e., no wrist-grounding straps or conductive and grounded table surfaces). In these environments, people are likely to touch I/O connector pins during the connecting and disconnecting of cables.
Conventional methods of shunting ESD energy to protect ICs involves devices such as Zener diodes, metal oxide varistors (MOVs), transient voltage suppression (TVS) diodes, and regular complementary metal oxide semiconductor (CMOS) or bipolar clamp diodes. However, at the much higher data rates of USB 2.0, IEEE 1394, and digital visual interface (DVI), the parasitic impedance of traditional protection devices can distort and deteriorate signal integrity.
FIG. 1 shows a configuration of diodes used to protect multiple signals. Diodes 12 are used to clamp a number of signals 14 to a power rail 100, while diodes 16 clamp the signals 14 to ground 102. A Zener diode 18 provides over-voltage discharge protection between power 100 and ground 102. As can be seen in the characteristic 140, voltage swings are clamped such that negative signal swing is limited to one diode drop. Although, larger negative swings can be obtained by using one of the channels as ground as shown in FIG. 2, this approach results in high crosstalk between channels due to the resultant high impedance to ground pin.
Moreover, conventional transient voltage suppressors are fabricated using processes that do not permit customization or significant alteration of Zener breakdown voltage. A conventional back-to-back transient voltage suppressor may have a performance specification in which the forward and backward breakdown voltages are non-symmetrical and difficult to change within the confines of the technology used in that product. Moreover, the failure of conventional devices to provide symmetry limits applicability in many applications. Generally, the symmetry and breakdown voltages of devices in conventional systems may be altered only at the cost of increased layout inefficiency efficient.